Accurate link budgeting for submarine cables requires detailed knowledge of the dispersion map and per-wavelength power out of each amplifier (power profile), which may vary from span to span along the link (gain evolution). In practice, complete dispersion and manufacturing data for deployed cables is often not available to third party terminal equipment suppliers or cable owners, and it is only possible to measure the power profile at the termination points of the link. Coherent Optical Time Domain Reflectometers (C-OTDRs) are used to measure loss along a link.
For submarine cables that incorporate High Loss Loop Back (HLLB) channels with filters that reflect a portion of the amplifier output at a specific test channel wavelength into the return path, it is possible to measure the amplifier output at that test channel wavelength using C-OTDR. Note that this is only applicable to wavelengths that are reflected into the return path and cannot be used to determine the power profile at arbitrary wavelengths.
The present state of the art for link monitoring is summarized in K. Toge and F. Ito, “Recent research and development of optical fiber monitoring in communication systems”, Photonic Sensors, vol. 3, no. 4, pp. 304-313, 2013. The primary diagnostic used on submarine cables is the coherent OTDR (C-OTDR) which measures loss as a function of distance within each span. This technique is very useful for detecting fiber breaks.
The cumulative dispersion at the end of an optical link is reported by the WL3 coherent modems commercially available from Ciena Corporation headquartered in Hanover, Md., USA.
H. Onaka, K. Otsuka, M. Hideyuki and T. Chikama, “Measuring the Longitudinal Distribution of Four-Wave Mixing Efficiency in Dispersion-Shifted Fibers”, IEEE Photonics Technology Letters, vol. 6, no. 12, p. 1454, December 1994 describes pump/probe techniques for measuring the zero dispersion wavelength as well as the nonlinear refractive index n2 of an optical fiber (given the effective area of the fiber) by measuring the variation in four wave mixing (FWM) efficiency as a function of wavelength separation between continuous wave pump and probe wavelengths. FWM efficiency is maximized at the zero dispersion wavelength and the wavelength dependent periodicity of the FWM efficiency is related to the chromatic dispersion.
Pump/probe techniques for measuring the zero dispersion wavelength have been published where the spatial overlap of forward propagating probe pulses with backward propagating pump pulses at a different wavelength is observed through the production of four wave mixing products at inter modulation frequencies. This technique would be difficult to employ on submarine cables where counter propagating waves within each span are blocked at each repeater site.
M. Ohashi, “Fiber Measurement Technique Based on OTDR”, “Current Developments in Optical Fiber Technology”, Dr. Sulaiman Wadi Harun (Ed.), ISBN: 978-953-51-1148-1, reports using the Rayleigh scattering efficiency to extract the mode field diameter and dispersion. These techniques generally rely on averaging measurement in both directions and require precise measurement of the back scattered power which will be difficult to measure when the scattered light has to return to the transmission site through hundreds of amplifiers.
Most non-destructive polarization-dependent loss (PDL) and polarization-mode dispersion (PMD) techniques report the value accumulated over the length of the optical link. A polarization resolved OTDR (P-OTDR) which measures the polarization state of the back-scattered light is described in A. Galtarossa and L. Palmieri, “Spatially Resolved PMD Measurements”, Journal of Lightwave Technology, vol. 22, no. 4, p. 1103, 2004. The dynamic range of P-OTDR limits its reach to several kilometers with a spatial resolution of roughly half a meter. This technique is not applicable to multi-span systems because of the difficulty detecting the polarization state of the weak back-scattered signal and disturbance of the scattered light polarization state caused by PMD in the return path. P-OTDR is not able to resolve the circularly polarized component of the birefringence vector as the effect on the forward propagating pulse is canceled when the back-scattered light propagates through the same optical path in the reverse direction.
The hinge method described in L. E. Nelson, C. Antonelli, A. Mecozzi, M. Birk, P. Magill, A. Schex, and L. Rapp, “Statistics of Polarization dependent loss in an installed long-haul WDM system”, Optics Express, vol. 19, no. 7, p. 6790, 2011, can be used to infer the magnitude and number of PDL and PMD sections separated by hinges along an optical link. In this model, PDL activity is assumed to originate from a finite number of hinges that are distributed along the fiber, and are separated by spans with slowly varying PMD. At a given time instant the different channels experience the same PDL elements with different polarization rotations in between them. The PDL measurements at the end of the optical link for multiple wavelengths are combined into a single PDF. By fitting to the PDF it is possible to estimate the number of hinge elements and their strengths. A similar technique can be used to identify PMD hinges.
A simulation technique based on calculating the nonlinear interaction between pump and probe pulses propagating at different wavelengths is described in Y. Cao, W. Yan, Z. Tao, L. Li, T. Hoshida and J. Rasmussen, “A fast and accurate method to estimate XPM impact under PMD”, in 10th International Conference on Optical Internet (COIN), Yokohama, Japan, 2012. They calculate the nonlinear rotation matrix between the pump and probe pulses at the end of each span. This matrix accounts for the action of cross-phase modulation (XPM) and cross-polarization modulation (XPolM) in the span. The intent of that paper is to develop a simulation tool. The paper compares their model's predictions to those of split step Fourier simulations. There is no discussion of methods for applying the ideas of the paper to an experimental measurement.